1. Field of the Invention
The present invention relates to semiconductor radiation detectors and, more particularly, to the configuration of an electrode formed on at least one surface of the semiconductor radiation detector.
2. Description of Related Art
Cathode electrodes and anode electrodes of Cadmium Zinc Telluride (CZT) radiation detectors are presently formed of platinum thin films. The platinum forming each electrode is deposited via a suitable thin film deposition technique, such as, without limitation, sputtering, to a thickness of approximately 1000 angstroms. However, this thickness is not to be construed in a limiting sense.
Each platinum electrode defines an electronic contact that enables the CZT bulk material to have superior performance when acting as an x-ray or gamma-ray radiation detector and to interface with electronics with sufficient connectivity to allow for data acquisition and control electronics to form a functional and high performance CZT detector based sensor.
In the case of a CZT detector based sensor system, platinum electrodes provide for the proper work function and chemical bond to the CZT bulk material such that the work function matches the CZT bulk material's bandgap, resistivity, and interface electronic properties, thereby enabling the CZT bulk material, or other suitable compound semiconductor crystal material, to realize full depletion of the entire CZT bulk material volume with excellent collection of charge carriers generated by the absorbed x-rays and gamma rays. The high sensitivity provided by the full depletion of the entire CZT bulk material volume and the good charge collection ensures nearly loss free signal generation from absorbed photons and provides higher performance levels of CZT based detectors than many other semiconductor detectors.
In the manufacture of compound semiconductor devices, such as CZT detectors, the mechanical fragility of the delicate thin platinum electrode metal prohibits the use of robust manufacture methods to fabricate these devices. Specifically, after deposition, the delicate thin-film platinum electrodes are easily damaged during subsequent device fabrication steps such as, without limitation: in-line probe testing; device handling during downstream device processing; and bonding to interconnect substrates or read-out electronics during integration to signal processing electronics.
One means of compensation for the fragility of thin platinum electrodes involves the subsequent deposition of an excessively thick second pure metal layer (typically gold) using a suitable thick film deposition technique and, if desired, a third pure metal layer (typically vanadium) using a suitable thick film deposition technique to protect the underlying platinum layer and provide a (deformable) buffer layer between the underlying platinum layer and probe contacts, bonding bumps, in-line device handling tools, and the like. The application of one or more excessively thick film metal layers, however, is impractical and costly with thin-film equipment, whereupon this approach requires the implementation of alternative thick-film deposition processes into the CZT detector fabrication process. With excessive film thickness, however, there is also the confounding relationship that film conformality to its substrate is reduced, thus resulting in the potential for electrodes to delaminate from the underlying surface. Excessive electrode thickness is also not desirous from a device reliability perspective.
The current state of the art in the fabrication of CZT detectors, and other compound semiconductor devices, has not produced devices with acceptably robust electrode structures which are capable of being mechanically probed prior to bonding of device pixel and full-area electrodes to interconnect substrates or read-out electronics. Existing CZT detectors are frequently damaged when probed in-process, thus requiring repeated and excessive processing to allow for such processing. An illustration of induced pin-probe damage on prior art platinum electrodes is shown in FIGS. 1A and 1B.
The performance and cost of CZT detectors, and other compound semiconductor devices, would greatly benefit from in-process probe testing thereof before proceeding further in the manufacturing cycle. This would provide for opportunities to improve production efficiency and reduce manufacturing costs for the fabrication of these detectors. For example, knowing ahead of the completed/mounted CZT detector manufacture that the CZT detector is either functional, non-functional, or partially functional, would enable decisions to be made to further process, reject, or selectively bin the CZT detector by quality level for subsequent final device manufacture disposition, respectively.
After deposition on a CZT detector, platinum electrodes are soft and frequently scratched during subsequent handling and manufacture of CZT detectors into radiation detectors. Even minor scratching of these delicate platinum electrodes can cause damage to the underlying CZT-to-metal interface and the generation of electronic noise in use of the CZT detector. At a minimum, this damage can deteriorate the performance and reduce the operability range (bias voltage, temperature) of the CZT detector. Often this damage is catastrophic and renders the CZT detector unusable for its intended application. When scratched, a CZT detector may require rework, re-metallization, or may be lost as scrap. Often the weight of a CZT detector on a so-called clean room wipe is all that is required to damage these delicate platinum electrodes alone or platinum electrodes having one or more suitable overlayers, such as, without limitation, a gold overlayer, or the combination of gold and vanadium overlayers.
Another area where more robust platinum electrodes would greatly improve fabrication yields and the short and long-term reliability of CZT detectors is during the bonding of the CZT detectors to interconnect substrates or read-out electronics. Typically, some form of flip-chip bonding is used to integrate CZT detectors to readout electronics (e.g., conductive epoxy bump bonding, low temperature solder bonding, gold stud bonding, ball-grid arrays, unidirectional conductive epoxy bonding and the like). All of these interconnect technologies employ dissimilar materials and wide curing/processing temperature cycles to achieve the required electrode/pixel connectivity. Thermo-mechanical stresses induced during the bonding process and long-term mechanical fatigue as a result of thermal cycles are the two main causes of connectivity failure. In case of delicate compound semiconductors, such as CZT, metal film damage can occur to the semiconductor-to-metal interface of a platinum electrode during both the bonding process, as a result of applied forces and thermo-mechanical stresses, and in the field during use due to thermal cycling induced interface fatigue and failure.
Hardening the platinum electrodes or multi-layer electrode described above would reduce device failure during the bonding process and improve the long-term reliability of the CZT detector in the field. Moreover, a hardened platinum layer by itself or in combination with a second and, if desired, a third layer would serve as a stress and strain barrier against mechanical deformation during the bonding process and subsequent use in the field while maintaining the favorable electrical properties of the barrier at the semiconductor-platinum interface.
However, common methods of work hardening, such as thermal processing to create tempering, are not permissible processes by which to improve electrode mechanical durability for many compound semiconductor crystals. For example, the detector material CZT is mechanically sensitive and work hardening is not a viable option for a platinum electrode of a CZT detector. CZT material is also thermally sensitive to temperatures above approximately 200° C. and such temperatures may not be employed as the device performance degrades due to thermally induced material changes. This eliminates bulk thermal processing as a method by which mechanical characteristics of the platinum may be improved.